Fun Brain offers a wide range of online educational games for children for grades K through 8th, a teacher's section is also available. No fee or membership required.
Greta's Game Station Wow! I have never seen such an extensive list of interactive games for every subject.
Show Me contains interactive sites from various museums in the UK.
Khan Academy has a large selection of online videos in various topics however, there appears to be more high school level math and science than anything else. It also includes test prep for the SAT and the CAT.
Sheppard Software interactive games for various subjects and grades plus.
Math Angie while the name screams math there are games for various subjects and age/grades.
EduPup also includes games for various subjects and grade/ages.
Free Online Learning very unique site includes over 90 topics a lot I have not seen on other sites such as a game for paying bills.
Freerice has limited topics, but is a great site for testing knowledge, I had fun playing and learning.